This disclosure is related to the field of neutron-induced radiation measurements for determining petrophysical properties of formations such as subsurface formations traversed by a wellbore. More specifically, the disclosure relates to using neutron induced gamma ray measurements to determine one or more petrophysical parameters of such formations.
Various neutron-induced radiation measurements have been used to evaluate characteristics of subsurface formations from a wellbore since as early as the 1950s. Neutrons can interact with subsurface formations in different ways. They can be scattered elastically, which means kinetic energy and momentum are conserved; they can be scattered inelastically, which means certain nuclei go into an excited state while kinetic energy is lost; they can also be captured by a nucleus to form a new nucleus; it is also possible that the neutron interaction causes a nuclear reaction resulting in the emission of one or more nucleons from the target nucleus. The probability of a neutron interacting with a nucleus is measured by the respective interaction cross section, which is a function of many parameters, such as incident neutron energy, outgoing neutron energy (if a neutron emerges from the interaction), scattering angle, interaction type and interactive nucleus type, among others. Thus, neutrons can enable measurement of many different formation properties due to the variety and complexity of their interactions.
One important wellbore neutron measurement known in the art is the neutron porosity measurement. The basic principle of such measurement is to impart high energy neutrons (typically several million electron volts—“MeV”—depending on the neutron source type) into the formation and to measure the thermal (or epithermal) neutron flux at one or more certain distances from the neutron source. The one or more detectors can be either neutron detectors such as helium-3 proportional counters, or gamma ray detectors such as scintillation counters. Gamma ray detectors measured neutron induced gamma rays as an indirect measurement of the neutron flux. In many neutron porosity well logging instruments, a ratio of detected radiation event (count) rate between a detector spaced from the neutron source at a first axial distance with respect to the detected count rate at a second, longer axial spacing from the source is used to determine neutron porosity.
The neutron porosity measurement is very sensitive to the hydrogen content in the formation because hydrogen is the most effective neutron moderator among all elements found in earthen formations. High hydrogen content can slow down neutrons to thermal energy (0.025 eV at room temperature) before they can travel an appreciable distance. Thus, HI (Hydrogen Index) and porosity (fresh water-filled) may be readily determined from such measurements. Therefore, using numbers of detected radiation events related to numbers of thermal neutrons has proven effective in determining formation porosity. A limitation of the neutron porosity measurement is that it is accurate only for fresh water-filled, clean (clay free) single lithology (such as sandstone, limestone and dolomite) formations. Some other environmental conditions need special treatment, such as gas-filled porosity, shale, and complex lithology. In addition, thermal neutron porosity measurement is sensitive to various environment effects including temperature and borehole and formation salinity.
It is more difficult to measure HI or neutron porosity based on measurements from gamma ray detectors as compared to measurements from neutron detectors. Gamma ray detectors measure the “prompt” gamma rays from neutron capture interaction in formation, wellbore or the well logging instrument. One may define “neutron-neutron porosity” as neutron porosity based on a neutron source and neutron detector(s) and may define “neutron-gamma porosity” as neutron porosity based on a neutron source and gamma ray detector(s). The physics of measuring “neutron-neutron porosity” only relates to neutron transport within the well logging instrument, the wellbore and the formations. The physics of measuring “neutron-gamma porosity” relates to both neutron and gamma ray transport, so that it is more complex. Thus, neutron-gamma porosity typically is more susceptible to environmental effects, and such measurements may be more difficult to interpret than neutron-neutron porosity measurements.
On the other hand, there may be advantages associated with measuring neutron-gamma porosity. The detected radiation event (count) rate of a gamma ray detector can be more than one order of magnitude higher than for a neutron detector given similar neutron source energy and neutron source output and relative detector spacing from the neutron source. The radial depth of investigation of neutron well logging instrument using one or more gamma ray detectors is typically deeper than for a similarly configured neutron-neutron well logging instrument. The energy of a prompt gamma ray from neutron capture is normally in the MeV range. Such high energy gamma rays can travel a longer distance in medial surrounding the well logging instrument than a thermal neutron before capture by atomic nuclei of certain materials in the formations. A gamma ray detector can also provide gamma ray spectroscopy and inelastic-gamma ray-based gas measurements, which neutron well logging instrument using thermal or epithermal neutron detectors, e.g., helium-3 proportional counters, cannot. The foregoing possible advantages make neutron-gamma porosity desirable to perform.
A limitation inherent to measuring neutron-gamma porosity is its loss of porosity sensitivity compared to neutron-neutron porosity, especially when formation fractional volume of pore space (porosity) exceeds about 30% (30 p.u.). A gamma ray near-spaced/far-spaced detector count rate ratio from a neutron-gamma porosity well logging instrument using conventional gamma ray detector analysis may exhibit negative sensitivity to porosity above about 30 p.u. In other words, a gamma ray detector count rate ratio can change in magnitude one way with respect to porosity between 0 to about 30 p.u., then it will change in the opposite way in a range from about 30 p.u. to 100 p.u. The relationship of gamma ray counting rates is therefore not a monotonic function of porosity.